The present invention is broadly directed to a plug closure for a medical implant such as a bone screw. More particularly, it is concerned with an exteriorly threaded plug which further has a threaded center bore that receives a set screw.
Bone screws are often employed in surgically implanted osteosynthesis apparatus, especially for fastening rods and transverse connectors to the spinal vertebrae for the purpose of stabilizing and/or adjusting spinal alignment. While both closed and open-ended types of bone screws are known, open-ended screws are particularly well suited for rod and connector arm installation and adjustment, because the rod does not have to be threaded through a bore, but rather can be laid or urged into an open receiver for the rod.
Such open-ended bone screws include a threaded shank coupled with a pair of upright branches which form a yoke having a U-shaped slot or groove for receiving a rod. Hooks and connectors also sometimes include open ends for receiving rods or the like. The rod is placed in the groove in generally perpendicular relation to the shank, and the open end of the yoke is closed off by a closure device. The closure device includes structure that is tightened against the rod or the like to hold the rod in place against the bottom of the groove. The closure device must hold and firmly secure the rod, in order to prevent rotational or translational movement of the rod relative to the bone screw following installation.
Prior art closure devices have varied in type and one of those types is a simple plug. That is, the interior walls of the branches are threaded and a similarly threaded plug is screwed into the bone screw between the branches to capture and hold the rod in the groove and between the branches. Because the branches must be spaced at least as wide as the rod, the plug must also be at least as wide, if not wider, than the rod. The plug must also not disassemble accidentally from the bone screw during use and the rod must be held securely enough by the plug to prevent rotation and axial relative movement between the rod and bone screw.
In order to perform the functions required of it, the plug should be tightly torqued relative to the bone screw. If the rod were linear, this could be accomplished with some difficulty. However, this is very seldom the case. In particular, the rod is almost always bent at each bone screw in a plane extending from front to rear in the patient in order to correctly position the rod for normal curvature of the back. Because the rod is bent, it does not rest flat against the bottom of the bone screw receiving groove, but tends to be raised one or both opposite ends. After installation, when the back is bent by the patient during activity, the rod will flex slightly about the bone screw. Consequently, even if the plug is torqued to a selected torque (for example, 100 inch pounds is commonly used), the plug may have substantially less or no torque when the rod is flexed, so as to allow the rod to slip. This is because the plug is comparatively wide and rests on the raised portions of the rod. As the rod bends and the raised portions subside, the plug becomes loose.
While a plug is very simple and thus desirable to use, a construction is needed to keep the plug and rod from becoming loose and allowing the rod to move relative to the bone screw, especially when the rod is flexed.